3.Human Physiology
3.1 .Circulation
Circulation
Dr V Malathi
The Circulatory System
The circulatory system is also known as the cardiovascular system.
The chief function of the circulatory system includes transporting blood, nutrients, oxygen, carbon dioxide, and hormones throughout the body. It consists of the heart, blood vessels (arteries, veins, and capillaries), and blood.
Key Components of the circulatory system
- Heart: A muscular organ that pumps blood throughout the body.
- Arteries: Blood vessels that carry oxygenated blood away from the heart.
- Veins: Blood vessels that carry deoxygenated blood back to the heart.
- Capillaries: These tiny blood vessels permit the exchange of oxygen, carbon dioxide, and other substances between blood and tissues.
Major Functions of the circulatory system:
- Oxygen and Nutrient Transport: Delivers oxygen and nutrients to cells.
- Waste Removal: Removes carbon dioxide and other metabolic wastes from cells.
- Hormone Distribution: Transports hormones to target organs and tissues.
- Temperature Regulation: Helps maintain body temperature by distributing heat.
The Heart
- The heart is a muscular organ that pumps blood through the three divisions of the circulatory system namely
- the coronary (vessels that serve the heart),
- pulmonary (heart and lungs), and
- systemic (systems of the body)
Structure of the Heart
The myocardial, endocardium, and epicardium are the three layers that make up the heart.
The endocardium is the lining that lines the inside of the heart.
The middle layer of the heart wall and the majority of its heart muscle cells make up the myocardium.
The pericardium, a layered structure made of membranes that surrounds and protects the heart, is the second layer of cells that make up the epicardium.
This layer gives the heart enough room for vigorous pumping while also keeping it stationary to lessen friction between the heart and other structures.
The heart contains four chambers namely:
- Right Atrium: Receives deoxygenated blood from the body.
The right atrium receives deoxygenated blood from the superior vena cava, which drains blood from the jugular vein that comes from the brain and from the veins that come from the arms, as well as from the inferior vena cava which drains blood from the veins that come from the lower organs and the legs. In addition, the right atrium receives blood from the coronary sinus which drains deoxygenated blood from the heart itself.
- Right Ventricle: Pumps deoxygenated blood to the lungs.
The deoxygenated blood from heart then passes to the right ventricle through the atrioventricular valve or the tricuspid valve, a flap of connective tissue that opens in only one direction to prevent the backflow of blood. The biscuspid or mitral valve separates the chambers on the left side of the heart.
After the right ventricle is filled, it pumps the blood through the pulmonary arteries. This blood passes through the semilunar valve (or pulmonic valve) and is taken to the lungs for re-oxygenation.
After blood passes through the pulmonary arteries, the right semilunar valves close preventing the blood from flowing backwards into the right ventricle.
- Left Atrium: Receives oxygenated blood from the lungs.
- The left atrium then receives the oxygen-rich blood from the lungs through the pulmonary veins. This blood passes through the bicuspid valve or mitral valve (the atrioventricular valve on the left side of the heart) to the left ventricle
Left Ventricle: Pumps oxygenated blood to the body.
From the left ventricle the blood is pumped out through aorta.
The aorta is the major artery of the body, taking oxygenated blood to the organs and muscles of the body.
Once blood is pumped out of the left ventricle and into the aorta, the aortic semilunar valve (or aortic valve) closes preventing backward flow of blood into the left ventricle.
This pattern of pumping is referred to as double circulation and is found in all mammals.
The pumping action of the heart is a function of the cardiac muscle cells, or cardiomyocytes,
These cardiomyocytes make up the heart muscle.
Cardiomyocytes, are striated muscle cells. They pump rhythmically and involuntarily. They are connected by intercalated disks exclusive to cardiac muscle.
The Cardiac Cycle
The repeating sequence of pumping of blood by the heart is called Cardiac cycle ,
Electrical signals cause the heart muscles to contract and relax. This causes the filling and emptying of the heart with blood
The human heart beats over 100,000 times per day.
In each cardiac cycle, the heart contracts ( systole), pushing out the blood and pumping it through the body;
This is followed by a relaxation phase ( diastole), where the heart fills with blood.
The atria contract at the same time, forcing blood through the atrioventricular valves into the ventricles.
The closing of the atrioventricular valves produces a “lup” sound.
Following a brief delay, the ventricles contract at the same time forcing blood through the semilunar valves into the aorta and the artery transporting blood to the lungs (via the pulmonary artery).
Closing of the semilunar valves produces a “dup” sound.
Blood Vessels
- There are three types of blood vessels: the arteries, veins, and capillaries.
- Arteries
Arteries are blood vessels that carry oxygenated blood away from the heart to various parts of the body.
Layers of Artery Walls include :
There are three distinct layers, or tunics, that form the walls of blood vessels; namely
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- Tunica Intima: The innermost layer, consisting of endothelial cells and elastic fibers.
- Tunica Media: The middle layer, made up of smooth muscle cells and elastic fibers, which helps control the diameter of the artery.
- Tunica Externa: The outermost layer, composed of elastic fibers and collagen, providing structure and support.
Types of Arteries:
-
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- Elastic Arteries: These are closer to the heart (e.g., the aorta) and contain more elastic fibers to handle the high pressure of blood pumped from the heart.
- Muscular Arteries: These are further from the heart and contain more smooth muscle tissue, allowing them to regulate blood flow by constricting or dilating.
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Major Arteries
- Aorta: The largest artery in the body, originating from the left ventricle of the heart and branching into smaller arteries.
- Coronary Arteries: Supply blood to the heart muscle itself.
- Carotid Arteries: Supply blood to the brain, neck, and face.
- Femoral Arteries: Supply blood to the lower limbs.
Functions of the arteries
- Oxygen and Nutrient Transport: Arteries carry oxygenated blood and nutrients to tissues and organs.
- Blood Pressure Regulation: The smooth muscle in the tunica media helps regulate blood pressure by adjusting the diameter of the arteries.
- Pulse: The expansion and contraction of elastic arteries with each heartbeat create the pulse that can be felt in various parts of the body.
Arterial Circuits:
- Systemic Circuit: Carries oxygenated blood from the heart to the body.
- Pulmonary Circuit: Carries deoxygenated blood from the heart to the lungs for oxygenation
Test your Understanding about the Circulatory system by attempting the interactive exercises below
